Historically, halide-containing raw materials, such as, SiCl.sub.4 or mixtures of SiCl.sub.4 with various dopants, have been used in the manufacture of preforms by vapor phase deposition techniques, such as, the MCVD (modified chemical vapor deposition), VAD (vapor axial deposition), and OVD (outside vapor deposition) techniques.
In the MCVD technique, the halide-containing raw materials are vaporized and reacted with oxygen to form oxide particles which are deposited on the inside of a fused-silica tube. In the VAD and OVD procedures, vaporized, halide-containing raw materials are hydrolyzed in a burner to produce soot particles which are collected on a rotating starting rod (bait tube) in the case of VAD or a rotating mandrel in the case of OVD. In some OVD systems, the cladding portion of the preform is deposited on a previously-formed core preform, rather than on a mandrel.
Various vaporizers have been developed which can be used in such processes, examples of which can be found in Aslami, U.S. Pat. No. 4,212,663, Blankenship, U.S. Pat. No. 4,314,837, Tsuchiya et al., U.S. Pat. No. 4,938,789, and U.K. Patent Publication No. 1,559,978 (hot oil heating).
In particular, French, U.S. Pat. No. 4,529,427, discloses a vaporization system in which oxygen and at least one liquid reactant, e.g., SiCl.sub.4, are introduced into a heated chamber where the liquid reactant is flash vaporized. The resulting oxygen/reactant gas mixture is then used to produce the desired preform. Other flash vaporizers are disclosed in Japanese Patent Publication No. 58-125633 (see, in particular, FIG. 4) and in Antos et al., U.S. Pat. No. 5,078,092, where the material to be vaporized can be halide-free. Soubeyrand et al., U.S. Pat. No. 5,090,985, discloses the use of a horizontal thin film evaporator for vaporizing various raw materials employed in the preparation of coated glass articles.
The use of halide-containing raw materials generates substantial quantities of halide-containing by-products, e.g., hydrochloric acid. To avoid environmental pollution, these by-products must be collected, which increases the overall cost of the preform production process. Accordingly, halide-free materials and, in particular, halide-free, silicon-containing materials are desirable starting materials for the production of preforms. See Dobbins et al., U.S. Pat. No. 5,043,002.
As explained in the Dobbins et al. patent, the relevant portions of which are incorporated herein by reference, particularly preferred halide-free, silicon-containing materials for use in producing preforms are polymethylsiloxanes, with the polymethylcyclosiloxanes being particularly preferred, and with octamethylcyclotetrasiloxane (OMCTS) being especially preferred. These same halide-free, silicon-containing raw materials are preferred for use with the present invention.
Cain et al., U.S. Pat. No. 5,356,451, discloses a vaporizer specifically developed for use with halide-free, silicon-containing raw materials, such as OMCTS. In accordance with this technology, a preheater in series with a flat plate vaporizer is used to change the state of the raw material from a liquid to a vapor. Both the preheater and the flat plate vaporizer use electrical resistance wires as their source of heat. In the preheater, the liquid is heated to below its boiling point. In the flat plate vaporizer, additional heat is added to (1) bring the fluid to its boiling temperature and (2) provide the necessary energy to overcome the material's latent heat of vaporization, whereupon the fluid becomes a vapor.
Halide-free, silicon-containing materials are difficult to vaporize due to their sensitivity to cracking and polymerization when subjected to excessive temperatures. Polymerization results in the production of higher molecular weight species which form gums and gels. Additionally, higher molecular weight species may exist in the raw materials as originally manufactured. Such species do not easily vaporize and over time the polymerized materials tend to foul heat transfer surfaces and plug piping systems. Current data suggest that it may not be possible to produce vapor from materials such as OMCTS without incurring some polymerization.